1. Field of the Invention
The present invention relates to a system which supplies gas enriched with nitrogen more than air to an aircraft fuel tank.
2. Description of the Related Art
Since an aircraft fuel tank is filled with vaporized fuel during a flight, it is necessary to prevent explosion of the fuel tank when, for example, the fuel tank is struck by lightning or a short occurs in the wiring. Thus, there has been proposed an explosion-proof system which supplies nitrogen enriched air (referred to as NEA below) having a higher nitrogen concentration and a lower oxygen concentration than air to the fuel tank.
While the air has an oxygen concentration of about 21%, the oxygen concentration of the NEA is set to, for example, 12% or less. To produce the NEA, an air separation module (ASM) that uses a permselective membrane composed of high molecules having different permeability coefficients for oxygen molecules and nitrogen molecules is employed. Bleed air from a flight engine is used as a supply source of air to be supplied to the ASM.
U.S. Pat. No. 6,547,188 proposes a process for supplying NEA to an aircraft fuel tank.
U.S. Pat. No. 6,547,188 employs the concentration of inert gas in the NEA, and the supply of the NEA to the fuel tank as a target of control. In U.S. Pat. No. 6,547,188, the two control targets are adjusted according to a flight phase from takeoff to landing of the aircraft. In U.S. Pat. No. 6,547,188, the flight phase is divided into a phase from takeoff until entering a descent phase for landing through an ascent phase and a cruising phase (referred to as first phase below), and the descent phase (referred to as second phase below).
In U.S. Pat. No. 6,547,188, a relatively small amount of NEA is supplied in the first phase, and a relatively large amount of NEA is supplied in the second phase. It is described in U.S. Pat. No. 6,547,188 that the supply of the NEA to the fuel tank is increased in the second phase so as to compensate for an increase in the atmospheric pressure since the altitude is lowered in the second phase.
The oxygen separation performance of the ASM used in the process for supplying the NEA to the aircraft fuel tank depends on the temperature and the pressure of the bleed air supplied to the ASM. For example, when the bleed air supplied to the ASM has a high temperature (about 180° F.), high oxygen separation performance is obtained. Thus, desired NEA with a high concentration can be produced even when more bleed air is supplied. Meanwhile, when the temperature of the supplied bleed air is lower, the oxygen separation performance is deteriorated. Thus, when the NEA with the same nitrogen concentration is to be obtained, the amount of production of the NEA is decreased. Therefore, the bleed air is normally supplied to the ASM with the temperature regulated so as to produce more NEA.
When the ASM is continuously used, the oxygen separation performance is deteriorated. It is thus necessary to replace the ASM at regular intervals. However, a permselective membrane constituting the ASM is expensive, and a replacement operation thereof is complicated.
The present invention has been made in view of such problems, and an object thereof is to provide an NEA supply system which can decrease the replacement frequency of an ASM.